Conventional single and twin screw extruders for laboratory use have been available from manufacturers. Technical literature describing such single and twin screw laboratory scale extruders includes: Leistritz Applications Bulletin, American Leistritz Extruder Corporation, 1983; Leistritz Information-Technical Bulletin 20, Leistritz Maschinenfabrik GmbH, 1980; Leistritz Information-Technical Bulletin 22, Leistritz Maschinenfabrik GmbH, 1980; and, Leistritz Extrusilnstechnik, Leistritz Maschinenfabrik GmbH, October 1979. The single and twin screw extruders described in the literature process materials at a rate of about 15 g/minute to more than 180 g/minute. The extruder barrels are heated and portions of the heated section include heaters coupled with forced cooling to regulate the amount of heat supplied thereto. The extruder screws are driven at one end. Another example of such extruders is described in Egan Extruders--Bulletin No. 202, Frank E. Egan & Company, October 1968.
In addition, there are a number of technical papers available which discuss general characteristics of twin screw extruders including: Chris J. Rauwendaal, "Analysis and Experimental Evaluation of Twin Screw Extruders," Polymer Engineering and Science, Volume 21, No. 16, November 1981, pages 1092-1100; William Thiele, "Expanding Uses for Counter-rotating, Intermeshing, Twin Screw Extruders," (undated), American Leistritz Extruder Corporation; and, William Thiele, "The Counter-rotating Twin Screw: An Alternative Machine Approach," reprint from Plastics Compounding (1981, Industry Media Inc.). Laboratory scale models of twin screw extruders are also described in these papers.
None of the literature discloses extruders including either an isolator section for isolating the cooled feed zone of the extruder barrel from the heated zone of the extruder barrel or a dual end drive system for driving both ends of the extruder screw.
Conventional extruders process material at rates between about 15 g/min and about 3300 g/min depending on process parameters and the size of the equipment. In the spinning of filaments, for example, the time required to reach steady state in the process is about 20-30 minutes. At a 15 g/min processing rate, approximately 300-450 g material would be consumed. Thus, conventional extruders are not suitable for efficient processing of small experimental quantities of material (.ltoreq.100 grams) requiring throughput rates less than about 5 g/min and preferably between about 0.05-1.5 g/min.
The obvious solution of scaling down conventional extruders to meet the small throughput requirements for processing experimental quantities of materials is not acceptable. One major problem with scaling down conventional extruders to process small quantities of material is controlling the large thermal gradient which is present along a very short length of the scaled-down extruder barrel. More specifically, the feed portion of the extruder, which constitutes about 1/5 of the total length of the barrel and includes the feed hopper, feed tube and a first feed zone, must be kept at a temperature less than about 80.degree. C. to prevent premature softening of the material. At the same time, the melting and processing portion (heated portion) of the extruder barrel, which constitutes about 1/2 or more of the total length of the barrel, should be kept at a temperature of at least 150.degree. C. and as high as 350.degree. C. depending on the polymer composition. Reducing the barrel length to about 10 inches or less, as in the instant invention, produces, in the transition portion of the barrel (the portion between the feed portion and the heated portion), a temperature gradient of about 550.degree. C./cm. Thus, with a conventional extruder design, too much heat will reach the feed portion of the barrel and cause the raw material in the feed zone to become viscous and tacky. As a result, the extruder screw cannot feed material through the barrel. Additionally, because of the premature softening, a gravity feed system cannot be employed to supply the extruder. The consequences of employing a force feed system to supply material to the extruder include greater complexity, increased cost and, possibly, added functional problems because of the small amount of material being processed. Conventional laboratory scale extruders solve this problem by increasing the extruder length to maintain the feed portion remote from the heating portion thus eliminating the large thermal gradient. However, an elongated extruder would be too long and bulky for convenient table top use and would require more processing material than would be normally available.
Another major problem with scaling down the conventional extruder is premature failure of the extruder screw. In operation, extruder screws encounter substantial torsional loads caused by the changing viscosity of the material as it travels along the length of the extruder barrel. In order to meet the experimental sample size processing requirements noted before, the extruder screw in a conventional scaled-down extruder will not only have a large length to diameter ratio (about 25:1) but will also be very thin (equal to or less than about 1/4 inch in diameter). Thus, driving the long thin extruder screw at one end only, as is conventional, in a scaled-down conventional extruder would result in premature failure caused by twisting and/or breaking of the screw. This problem would be amplified in the conventional laboratory scale extruders which use combined heating and cooling units to gradually control the heat supplied to the barrel.
We have invented an advantageous and improved extruder apparatus for processing small amounts of material. The invention eliminates the heat transfer problem associated with scaled-down conventional extruder apparatus. It also eliminates the problem of premature failure of extruder screws employed in scaled-down conventional extruder apparatus. Additionally, it provides an inexpensive compact bench-top extruder apparatus which is capable of processing material at rates less than about 5 g/min and preferably about 0.05 to 1.5 g/min to produce laboratory specimens. As an added feature, the apparatus employs a simple gravity feed system.
The uses of the extruders are not limited to extrusion of shaped products. Because of the size of the apparatus, it can be conveniently used as an injector for feeding material to another extruder or some other device. In addition, it can also be used to measure the viscosity of liquids using the following known equation: ##EQU1## where .DELTA.P= pressure drop over the length of the nozzle or die outlet; R= radius of the nozzle or die outlets; L=lengths of the nozzle or die outlet; Q= flow rate from the nozzle or die outlet; and .mu.= the viscosity of the liquid.
Since .DELTA.P,R,L and Q are measurable quantities, the viscosity (.mu.) can be calculated. Other uses are apparent from the detailed description of the invention included herein.